The hoatzin (Opisthocomus hoazin) is a remarkable bird, adapted for life in the wetlands and seasonally flooded habitats of Amazonia.  Physically, it is the size of a chicken, but adorned with a bright blue face, ruby-red eyes, and a feathered crest on its head, which looks like a punk-rock mohawk.  This, along with its slow unwieldy flight and its hoarse croaking calls, gives it a primitive appearance that roughly resembles artistic reconstructions of the famous Jurassic fossil bird Archaeopteryx.  Increasing this resemblance, the newly-hatched hoatzin chicks have claws on their wings, and use them to balance and to climb up the vegetation.  In terms of evolution, the hoatzin is the unique bird whose origin is directly associated with the development of the Amazon river system*.


Hoatzins live in large colonies in the quiet backwaters of Cantão State Park, where they play an important ecological role, transforming leafy vegetation into nutrients accessible to the aquatic ecosystem. The hoatzin is among the few birds that can survive on a diet of leaves. Leaves have little nutrition, lots of tannins and other secondary compounds that make digestion difficult.  To get enough nutrition out of them, the hoatzin evolved an enormously enlarged crop, which is so large that the bird's breast bone and flight muscles are displaced and reduced in size.  This, and the weight of the crop full of leaves, are the reason why the hoatzins display such an ungainly flight.  For up to 48 hours the leaves ferment in the crop, which contains symbiotic bacteria, breaking down the tannins and releasing the nutrients.  In Cantão, hoatzins feed mainly on the leaves of vines growing over flooded forest trees and shrubs, on floating vegetation, and on marsh grasses, for which they land on the ground and graze like geese.  Colonies of over one hundred birds occur in the park, sometimes in close proximity to one another.


In January, as the flood waters rise, the hoatzins move to the marshes on the inside of river curves and at the ends of oxbow lakes.  There, they nest among the branches of trees isolated by the flood waters from the nearby forest.  They build a platform of sticks over the water, safe from most predators.  The young hatch at the peak of the floods, in March and April.  Almost as soon as they hatch they leave the nest, and if a predator approaches, they sit very still in a dense thicket while the adult birds nearby fly about and call loudly, diverting the predator's attention.  


After a few days, the chicks are too large to be so inconspicuous, but then they have another fabulous trick: if a predator approaches, they drop into the water, swim under the surface for a few meters, and climb back up the vegetation when it is safe to do so, using their amazing wing claws. They do no return to their original nests, but are found by the adults which take care of them in the vegetation.






* Most bird families evolved once South America became an island, though none of these belong to aquatic or semi-aquatic families.

 

                     Cantão State Park, March 2011

                    HOATZINS

"The most notable product of bird evolution in the world's greatest river systems, the Amazon and Orinoco."

                                                                                                                                                      - Helmut Sick.

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